I don't know if KDE got it working yet, but Gamescope's works pretty well out-of-the-box. Nobara and maybe Chimara OS already have this ready with a session for Steam Big Picture mode.
Kinda funny that Windows games seem to always get compatibility with these things first. I guess just adding support in Wine means more games get the functionality at once than developers adding it on a per-game basis.
Not only that, you asked for their opinions.